The present invention relates to methods and apparatus for the treatment of metal-containing solutions. More particularly, the present invention relates to a continuous ion exchange method which is particularly useful for the selected concentration and removal of metals from aqueous solutions, either for pollution control or for resource recovery.
In various hydrometallurgical processes, aqueous solutions containing large amounts of dissolved mineral ions such as gold, silver, uranium, cobalt, molybdenum and copper are employed or produced. Additionally, in various plating plants, aqueous solutions containing relatively high concentrations of silver, cadmium, zinc, chromium, and copper result from the processes employed. Such aqueous solutions can be pollutants because of the relatively high levels of metal ions present. However, many of these metals are valuable in their own right and selective extraction of them from the aqueous solutions is usually highly desirable.
In U.S. Pat. No. 4,306,946, issued Dec. 22, 1981 in the name of the instant inventor, Donnan dialysis is employed as a continuous ion exchange process. This process employs a solid ion exchange membrane as a barrier between feed and eluant solutions. However, the method suffers from low removal rate which is determined by the diffusion rate of ions through the solid, ion-exchange membrane. However, it is nonetheless a continuous method and provides many advantages over conventional batch processes which require complex flow schemes and, consequently, a high equipment cost.
Another form of metal recovery process is described in the article "Coupled Transport Membranes" by R. W. Baker et al. in the Journal of Membrane Science, Vol. 2, pages 213-233, 1977. The process described therein employs a porous membrane which is impregnated with an organic absorbing medium. However, this process exhibits certain short-comings, particularly membrane instability. Membrane extraction performance deteriorates significantly due to the loss of the organic medium and also due to the filling of the pores with the aqueous solution. This happens when the organic medium is leached out of the pores or pushed out of the pores due to the pressure imbalance across the membrane. The present invention is distinct from the method described in this paper chiefly in that the liquid ion exchange material is not impregnated in a membrane but rather is disposed in a recirculating system between separate membranes in distinct fluid-tight chambers. Accordingly, the problem of the loss of ion exchange material is not present.
Additionally, a process for hydrometallurgical extraction is described in U.S. Pat. No. 3,957,504, issued May 18, 1976 to Ho et al. FIG. 4 of the patent to Ho et al. is particularly relevant in that it illustrates a hydrometallurgical extraction process flow diagram in which a chelating fluid is recycled back to a first ionic membrane extraction unit. However, no provision is made in the apparatus of Ho et al. for treatment of the recycled chelating solutions. In particular, there is no provision for the use of an aqueous/organic separator which the present inventor has found to be particularly effective in selectively isolating and enriching desired metallic species using porous membranes.